Gel electrophoresis was used to identify radioactive opsin extracted from the outer segments of dogs injected intravitreally with labeled amino acids. Concurrent autoradiography showed a migrating band of labeled opsin in the outer segments of both normal and affected littermates of miniature poodles carrying the gene for an inherited progressive retinal atrophy. In vitro incubation of bovine, frog, chick and dog retinas with labeled fucose has resulted in the labeling of an opsin-like protein that is distinct from rhodopsin and may be a cone pigment. Thus, fucose may be useful as a marker to follow the viability of cones in degenerating retinas such as in dogs.